Mat for mounting a pollution control element in a pollution control device for the treatment of exhaust gas

ABSTRACT

A pollution control device suitable for use with an internal combustion engine (e.g., a diesel engine) and comprising a pollution control element arranged in a casing with non-woven mat disposed between the casing and the pollution control element, said non-woven mat being a non-intumescent mat comprising at least 90% by weight based on the total weight of the mat of chopped magnesium aluminium silicate glass fibers that have a number average diameter of 5μ or more and a length of 0.5 to 15 cm, said glass fibers being needle punched or stitch bonded and said mat being free or substantially free of organic binder.

FIELD OF THE INVENTION

The present invention relates to pollution control devices that includea mounting mat for mounting a pollution control element in the pollutioncontrol device. In particular, the invention relates to pollutioncontrol devices that are intended for the treatment of the exhaust of aninternal combustion engine (e.g., a diesel engine). The mounting mat ofthe pollution control device can be designed so that it is particularlysuited for lower temperature applications such as diesel catalyticconverters or other pollution control elements adapted for reducingpollution from the exhaust of diesel engines.

BACKGROUND OF THE INVENTION

Diesel pollution control devices include catalytic converters and dieselparticulate filters or traps. The pollution control devices typicallycomprise a metal housing or casing with a pollution control elementsecurely mounted within the casing by a resilient and flexible mountingmat. Pollution control devices are universally employed on motorvehicles to control atmospheric pollution. Two types of devices arecurrently in widespread use: catalytic converters and diesel particulatefilters or traps. Catalytic converters contain a catalyst, which istypically coated on a monolithic structure mounted within a metallichousing. The monolithic structures are typically ceramic, although metalmonoliths have also been used. The catalyst oxidizes carbon monoxide andhydrocarbons and reduces the oxides of nitrogen in automobile exhaustgases to control atmospheric pollution.

Diesel particulate filters or traps are typically wall flow filters,which have honeycombed, monolithic structures typically made from porouscrystalline ceramic materials. Alternate cells of the honeycombedstructure are typically plugged such that exhaust gas enters in one celland is forced through the porous wall to an adjacent cell where it canexit the structure. In this way, the small soot particles that arepresent in diesel exhaust gas are collected.

The monoliths and in particular the ceramic pollution control monoliths,used in pollution control devices are fragile and susceptible tovibration or shock damage and breakage. They have a coefficient ofthermal expansion generally an order of magnitude less than the metalhousing which contains them. This means that as the pollution controldevice is heated the gap between the inside peripheral wall of thehousing and the outer wall of the monolith increases. Even though themetallic housing undergoes a smaller temperature change due to theinsulating effect of the mat, the higher coefficient of thermalexpansion of the metallic housing causes the housing to expand to alarger peripheral size faster than the expansion of the monolithicelement. Such thermal cycling occurs hundreds of times during the lifeand use of the pollution control device.

To avoid damage to the ceramic monoliths from for example road shock andvibrations, to compensate for the thermal expansion difference, and toprevent exhaust gases from passing between the monolith and metalhousing (thereby bypassing the catalyst), mounting mats are disposedbetween the ceramic monolith and the metal housing. These mats mustexert sufficient pressure to hold the monolith in place over the desiredtemperature range but not so much pressure as to damage the ceramicmonolith.

Many of the mounting mats described in the art have been developed formounting catalytic converters for treatment of exhaust from gasolineengines which typically operate at high temperature. Known mounting matsinclude intumescent sheet materials comprised of ceramic fibers,intumescent materials and organic and/or inorganic binders. Intumescentsheet materials useful for mounting a catalytic converter in a housingare described in, for example, U.S. Pat. No. 3,916,057 (Hatch et al.),U.S. Pat. No. 4,305,992 (Langer et al.) U.S. Pat. No. 5,151,253 (Merryet al.) U.S. Pat. No. 5,250,269 (Langer) and U.S. Pat. No. 5,736,109(Howorth et al.). In recent years, non-intumescent mats comprised ofpolycrystalline ceramic fibers and binder have been used especially forthe so-called ultra thin-wall monoliths, which have significantly lowerstrength due to their extremely thin cell walls. Examples ofnon-intumescent mats are described in, for example, U.S. Pat. No.4,011,651 (Bradbury et al.), U.S. Pat. No. 4,929,429 (Merry), U.S. Pat.No. 5,028,397 (Merry), U.S. Pat. No. 5,996,228 (Shoji et al.), and U.S.Pat. No. 5,580,532 (Robinson et al.). Polycrystalline fibers are muchmore expensive than normal, melt formed ceramic fibers and, therefore,mats using these fibers are only used where absolutely necessary as, forexample, with ultra thin-wall monoliths.

U.S. Pat. No. 5,290,522 describes a catalytic converter having anon-woven, mounting mat comprising at least 60% by weight shot-free highstrength magnesium aluminosilicate glass fibers having a diametergreater than 5 micrometers. The mounting mats taught in this referenceare primarily intended for use in high temperature applications as canbe seen from the test data in the examples where the mats are subjectedto exhaust gas temperatures of more than 700° C.

U.S. Pat. No. 5,380,580 describes a flexible non-woven mat comprisingshot-free ceramic oxide fibers selected from the group consisting of (a)aluminosilicate fibers comprising aluminum oxide in the range from 60 toabout 85% by weight and silicon oxide in the range of 40 to about 15% byweight silicon oxide, based on the total weight of saidaluminosilicate-based fibers, said aluminosilicate-based fibers being atleast 20% by weight crystalline (b) crystalline quartz fibers and (c)mixtures of (a) and (b), and wherein the combined weight of saidaluminosilicate-based fibers and said crystalline quartz fibers is atleast 50% by weight of the total weight of said non-woven mat. Theflexible non-woven mat can additionally comprise high strength fibersselected from the group consisting of silicon carbide fibers, siliconnitride fibers, carbon fibers, silicon nitride fibers, glass fibers,stainless steel fibers, brass fibers, fugitive fibers, and mixturesthereof.

Diesel Oxidation Catalysts (DOC's) are used on modern diesel engines tooxidize the soluble organic-fraction (SOF) of the diesel particulateemitted. Because of extremely low exhaust gas temperature, mounting ofDOC's with conventional mounting materials has been problematic. Theexhaust gas of modern diesel engines such as turbo-charged directinjection (TDI) engines may never exceed 300° C. This temperature isbelow the temperature needed to expand most intumescent mats. Theexpansion is needed to develop and maintain appropriate pressure withinthe catalytic converter. Additionally, this temperature is too low toburn out the organic binder contained in intumescent mat materials. Atthese temperatures the binder only softens, which acts to interfere withthe resiliency of the ceramic fibers. As a result, field failures haveoccurred with DOC's when using conventional intumescent mounting mats.To overcome these difficulties, the converters are sometimes heattreated prior to installation to expand the vermiculite and burn out thebinder. This is expensive and time consuming. Auxiliary wire mesh “L”seals have also been employed to augment the holding force ofintumescent mats at low temperature, but also add cost and complexity toassembly. Most non-intumescent mats while performing somewhat betterstill contain an organic binder, which significantly reduces theresiliency of the fibers in the 200-300° C. temperature range. This isexpected to be true for other diesel exhaust pollution control devices,as well, including lean NOx catalysts, Continuous regenerating traps(CRT's) and NOx traps.

U.S. Pat. No. 6,231,818 attempts to overcome the present difficulties ofmounting low-temperature, diesel catalysts by using non-intumescent matscomprised of amorphous, inorganic fibers. Although it is taught in thispatent that the mat can be organic binder free, it appears that severalof the mats used in the examples require the use of substantial amountsof binders. Moreover, it was found that the mounting mats disclosed inthis U.S. patent, still do not adequately perform for treatment ofexhaust from diesel engines, in particular TDI engines.

It was thus desirable to find an alternative mounting mat for mounting adiesel is pollution control monolith in the metallic casing of apollution control device for the treatment of exhaust from a dieselengine. In particular, it was a desire to obtain such improved mountingmats that can be manufactured in an easy and convenient way at anaffordable cost. Additionally, it was a desire to find mounting matsthat show good to excellent performance in at least one or more of thefollowing tests Real Condition Fixture Test (RCFT), Cyclical CompressionTest, and Hot Vibration Test. Desirably, the mounting mat is also moreacceptable in terms of health, safety and environmental aspects.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, there is provided anon-woven and non-intumescent mat comprising at least 90% by weightbased on the total weight of the mat of chopped magnesium aluminiumsilicate glass fibers that have a number average diameter of 5 μm ormore and a length of 0.5 to 15 cm, whereby the glass fibers are needlepunched or stitch bonded and the mat is free or substantially free oforganic binder. By ‘substantially free’ is meant that the amount ofbinder is not more than 1% by weight based on the weight of the mat,preferably not more than 0.5% by weight. The mat is used in a pollutioncontrol device for the treatment of exhaust from an internal combustionengine (e.g., a diesel engine) of a machine. The engine may be includedin a stationary machine such as for example in a power generator or in amotor vehicle. The mounting mat mounts a pollution control element(e.g., a diesel pollution control monolith) in a housing (e.g., ametallic casing) of the pollution control device and is typicallyarranged between the casing and the pollution control element.

According to a particular aspect, there is provided a mat for mounting apollution control element (e.g., a diesel pollution control monolith) ina housing (e.g., a metallic casing) of a pollution control device, themat being a non-intumescent mat comprising at least 90% by weight basedon the total weight of the mat of chopped magnesium aluminium silicateglass fibers that have a number average diameter of 5 μm or more and alength of 0.5 to 15 cm, the glass fibers being needle punched or stitchbonded, the mat being free or substantially free of organic binder andbeing comprised of at least two layers of the chopped magnesiumaluminium silicate glass fibers, wherein the two layers are differing intheir glass fiber composition. A mat according to this aspect was foundto be particularly suitable for optimizing performance and manufacturingcost of a mounting mat for pollution control devices for diesel engineexhaust.

In another aspect of the invention, there is provided a method oftreating exhaust gas from an internal combustion engine (e.g., a dieselengine) by subjecting the exhaust gas to a pollution control devicecomprising a pollution control element (e.g., a diesel pollution controlmonolith) arranged in a housing (e.g., metallic casing) with a non-wovenmat disposed between the casing and the pollution control element, saidnon-woven mat being a non-intumescent mat comprising at least 90% byweight based on the total weight of the mat of chopped magnesiumaluminium silicate glass fibers that have a number average diameter of 5μm or more and a length of 0.5 to 15 cm, said glass fibers being needlepunched or stitch bonded and said mat being free or substantially freeof organic binder.

With term ‘diesel pollution control monolith’ is meant a monolithicstructure that is suitable for and/or adapted for reducing the pollutioncaused by exhaust from a diesel engine and in particular includesmonolithic structures that are operative in reducing the pollution atlow temperatures, e.g. of 350° C. or less. Diesel pollution controlmonoliths include without limitation catalytic converters, dieselparticulate traps and NOx absorbers or traps.

The term ‘magnesium aluminium silicate glass fibers’ includes glassfibers that comprise oxides of silicon, aluminium and magnesium withoutexcluding the presence of other oxides, in particular other metaloxides.

BRIEF DESCRIPTION OF THE DRAWINGS

Solely for the purpose of illustration and better understanding of theinvention and without the intention to limit the invention in any waythereto, the following drawings are provided:

FIG. 1 is a perspective view of a catalytic converter of the presentinvention shown in disassembled relation.

FIGS. 2 and 3 show the results of the mats of Example 1 and ComparativeExamples 1 and 2 in a Real Condition Fixture Test.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Referring to FIG. 1 pollution control device 10 comprises metalliccasing or housing 11 with generally frusto-conical inlet and outlet ends12 and 13, respectively. Disposed within casing 11 is a diesel pollutioncontrol monolith 20 e.g., formed of a honeycombed monolithic body havinga plurality of gas flow channels (not shown) there through. Surroundingdiesel pollution control monolith 20 is mounting mat 30 comprising thechopped magnesium aluminosilicate glass fibers which serves to tightlybut resiliently support monolithic element 20 within the casing 11.Mounting mat 30 holds diesel pollution control monolith 20 in place inthe casing and seals the gap between the diesel pollution controlmonolith 20 and casing 11 to thus prevent or minimize diesel exhaustgases from by-passing diesel pollution control monolith 20.

The metallic casing can be made from materials known in the art for suchuse including stainless steel.

Examples of diesel pollution control monoliths for use in the pollutioncontrol device 10 include catalytic converters and diesel particulatefilters or traps. Catalytic converters contain a catalyst, which istypically coated on a monolithic structure mounted within a metallichousing. The catalyst is typically adapted to be operative and effectiveand low temperature, typically not more than 350° C. The monolithicstructures are typically ceramic, although metal monoliths have alsobeen used. The catalyst oxidizes carbon monoxide and hydrocarbons andreduces the oxides of nitrogen in exhaust gases to control atmosphericpollution. While in a gasoline engine all three of these pollutants canbe reacted simulataneously in a so-called “three way converter”, mostdiesel engines are equipped with only a diesel oxidation catalyticconverter. Catalytic converters for reducing the oxides of nitrogen,which are only in limited use today for diesel engines, generallyconsist of a separate catalytic converter. Suitable ceramic monolithsused as catalyst supports are commercially available from Corning Inc.(Corning N.Y.) under the trade name of “CELCOR” and commerciallyavailable from NGK Insulated Ltd (Nagoya, Japan) under the trade name of“HONEYCERAM”, respectively.

Diesel particulate filters or traps are typically wall flow filters,which have honeycombed, monolithic structures typically made from porouscrystalline ceramic materials. Alternate cells of the honeycombedstructure are typically plugged such that exhaust gas enters in one celland is forced through the porous wall to an adjacent cell where it canexit the structure. In this way, the small soot particles that arepresent in diesel exhaust gas are collected. Suitable diesel particulatefilters made of cordierite are commercially available from Corning Inc.(Corning N.Y.) and NGK Insulated Inc. (Nagoya, Japan). Dieselparticulate filters made of Silicon Carbide are commercially availablefrom Ibiden Co. Ltd. (Japan) and are described in, for example, JP2002047070A.

The magnesium aluminium silicate glass fibers used in the non-wovenmounting mat have an average diameter of at least 5 μm and a lengthbetween 0.5 and 15 cm, preferably between 1 and 12 cm. Preferably, theaverage diameter will be at least 7 μm and is typically in the range of7 to 14 μm. The glass fibers are preferably individualized. To provideindividualized (i.e., separate each fiber from each other) fibers, a towor yarn of fibers can be chopped, for example, using a glass rovingcutter (commercially available, for example, under the trade designation“MODEL 90 GLASS ROVING CUTTER” from Finn & Fram, Inc., of Pacoma,Calif.), to the desired length (typically in the range from about 0.5 toabout 15 cm). The fibers typically are shot free or contain a very lowamount of shot, typically less than 1% by weight based on total weightof fibers. Additionally, the fibers are typically reasonably uniform indiameter, i.e. the amount of fibers having a diameter within +/−3 μm ofthe average is generally at least 70% by weight, preferably at least 80%by weight and most preferably at least 90% by weight of the total weightof the magnesium aluminium silicate glass fibers.

The mat may contain up to 10% by weight of fibers other than magnesiumaluminium silicate glass fibers. Preferably however, the mat willconsist of only magnesium aluminium silicate glass fibers. If otherfibers are contained in the mat, they will typically be amorphous fibersand they should preferably also have an average diameter of at least 5μm. Preferably, the mat will be free or essentially free of fibers thathave a diameter of 3 μm or less, more preferably the mat will be free oressentially free of fibers that have a diameter of less than 5 μm.Essentially free here means that the amount of such small diameterfibers is not more than 2% by weight, preferably not more than 1% byweight of the total weight of fibers in the mat.

In a preferred method for making the nonwoven mat, the cut or choppedfibers can be separated by passing them through a conventional two zoneLaroche Opener (e.g., commercially available from Laroche S. A., Coursla Ville, France). The fibers can also be separated by passing themthrough a hammer mill, preferably a blow discharge hammer mill (e.g.,commercially available under the trade designation “BLOWER DISCHARGEMODEL 20 HAMMER MILL.” from C.S. Bell Co. of Tiffin, Ohio). Althoughless efficient, the fibers can be individualized using a conventionalblower such as that commercially available under the trade designation“DAYTON RADIAL BLOWER,” Model 3C 539, 31.1 cm (12.25 inches), 3horsepower from W. W. Grainger of Chicago, Ill. The chopped fibersnormally need only be passed through the Laroche Opener once. When usingthe hammer mill, they generally must be passed though twice. If a bloweris used alone, the fibers are typically passed through it at leasttwice. Preferably, at least 50 percent by weight of the fibers areindividualized before they are formed into a nonwoven mat.

Although cut or chopped fibers greater than about 15 cm are also usefulin preparing the nonwoven mat, they tend to be more difficult toprocess. Separation of the fibers tends to increase the loftiness (i.e.,decrease the bulk density) of the fibers making up the nonwoven matthereby lowering the density of the resulting mat.

To facilitate processing and separation of the chopped or cut fiberswith minimal breakage an antistatic lubricant (e.g., such as thatcommercially available under the trade designation “NEUTROSTAT” fromSimco Co. Inc., of Hatfield, N.J.) can be sprayed into the hammer millwhile the fibers are being separated.

The magnesium aluminium silicate glass fibers preferably comprisebetween 10 and 30% by weight of aluminium oxide, between 52 and 70% byweight of silicium oxide and between 1 and 12% of magnesium oxide. Theweight percentage of the aforementioned oxides are based on thetheoretical amount of Al₂O₃, SiO₂ and MgO. It will further be understoodthat the magnesium aluminium silicate glass fiber may contain additionaloxides. For example, additional oxides that may be present includesodium or potassium oxides, boron oxide and calcium oxide. Particularexamples of magnesium aluminium silicate glass fibers include E-glassfibers which typically have a composition of about 55% of SiO₂, 11% ofAl₂O₃, 6% of B₂O₃, 18% of CaO, 5% of MgO and 5% of other oxides; S andS-2 glass fibers which typically have a composition of about 65% ofSiO₂, 25% of Al₂O₃ and 10% of MgO and R-glass fibers which typicallyhave a composition of 60% of SiO₂, 25% of Al₂O₃, 9% of CaO and 6% ofMgO. E-glass, S-glass and S-2 glass are available for example fromAdvanced Glassfiber Yarns LLC and R-glass is available from Saint-GobainVetrotex.

According to a method for making the nonwoven mat, chopped,individualized fibers (preferably, about 2.5 to about 5 cm in length)are fed into a conventional web-forming machine (commercially available,for example, under the trade designation “RANDO WEBBER” from RandoMachine Corp. of Macedon, N.Y.; or “DAN WEB” from ScanWeb Co. ofDenmark), wherein the fibers are drawn onto a wire screen or mesh belt(e.g., a metal or nylon belt). If a “DAN WEB”-type web-forming machineis used, the fibers are preferably individualized using a hammer milland then a blower. Fibers having a length greater than about 2.5 cm tendto become entangled during the web formation process. To facilitate easeof handling of the mat, the mat can be formed on or placed on a scrim.Depending upon the length of the fibers, the resulting mat typically hassufficient handleability to be transferred to a needle punch machinewithout the need for a support (e.g., a scrim).

The nonwoven mat can also be made using conventional wet-forming ortextile carding. For wet forming processes, the fiber length ispreferably about 0.5 to about 6 cm. For textile processes, the fiberlength is preferably about 5 to about 10 cm.

A needle-punched nonwoven mat refers to a mat wherein there is physicalentanglement of fibers provided by multiple full or partial (preferably,full) penetration of the mat, for example, by barbed needles. Thenonwoven mat can be needle punched using a conventional needle punchingapparatus (e.g., a needle puncher commercially available under the tradedesignation “DILO” from Dilo of Germany, with barbed needles(commercially available, for example, from Foster Needle Company, Inc.,of Manitowoc, Wis.)) to provide a needle-punched, nonwoven mat. Needlepunching, which provides entanglement of the fibers, typically involvescompressing the mat and then punching and drawing barbed needles throughthe mat. The optimum number of needle punches per area of mat will varydepending on the particular application. Typically, the nonwoven mat isneedle punched to provide about 5 to about 60 needle punches/cm².Preferably, the mat is needle punched to provide about 10 to about 20needle punches/cm².

Preferably, the needle-punched, nonwoven mat has a weight per unit areavalue in the range from about 1000 to about 3000 g/m², and in anotheraspect a thickness in the range from about 0.5 to about 3 centimeters.Typical bulk density under a 5 kPA load is in the range 0.1-0.2 g/cc.

The nonwoven mat can be stitchbonded using conventional techniques (seee.g., U.S. Pat. No. 4,181,514 (Lefkowitz et al.), the disclosure ofwhich is incorporated herein by reference for its teaching ofstitchbonding nonwoven mats). Typically, the mat is stitchbonded withorganic thread. A thin layer of an organic or inorganic sheet materialcan be placed on either or both sides of the mat during stitchbonding toprevent or minimize the threads from cutting through the mat. Where itis desired that the stitching thread not decompose in use, an inorganicthread, such as ceramic or metal (e.g., stainless steel) can be used.The spacing of the stitches is usually from 3 to 30 mm so that thefibers are uniformly compressed throughout the entire area of the mat.

In accordance with a particular embodiment of the present invention, themat may be comprised of a plurality of layers of the magnesium aluminiumsilicate glass fibers. Such layers may be distinguished from each otherin the average diameter of the fibers used, the length of the fibersused and/or the chemical composition of the fibers used. Since the heatresistance and mechanical strength of fibers at temperature vary withtheir composition and to a lesser degree fiber diameter, fiber layerscan be selected to optimize performance while minimizing cost. Forexample, a nonwoven mat consisting of a layer of S-2 glass combined witha layer of E-glass can be used to mount a diesel catalytic converter. Inuse the S-2 glass layer is placed directly against the hotter, monolithside of the catalytic converter while the E-glass layer is against thecooler, metal housing side of the catalytic converter. The layeredcombination mat can withstand considerably higher temperatures than amat consisting of only E-glass fibers at greatly reduced cost comparedto a mat consisting of only S-2 glass fibers. The layered mats are madeby first forming the individual non-woven layers having a specific typeof fiber using the forming techniques described earlier. These layersare then needle bonded together to form the finished mat having thedesired discrete layers.

The mounting mats of the invention are particularly suitable formounting a diesel pollution control monolith in a pollution controldevice. Typically, the mount density of the mat, i.e., the bulk densityof the mat after assembly, should be at least 0.2 g/cm³ to providesufficient pressure to hold the monolith securely in place. At mountdensities above about 0.70 g/cm³ the fibers can be unduly crushed. Alsoat very high mount density there may be a risk that the monolith breaksduring assembly of the pollution control device. Preferably, the mountdensity should be between about 0.25 g/cm³ and 0.45 g/cm³. The pollutioncontrol device has excellent performance characteristics for use in lowtemperature applications such as in the treatment of diesel engineexhaust. The pollution control device may be used in a stationarymachine to treat the exhaust emerging from a diesel engine containedtherein. Such stationary machines include for example power sources forgenerating electricity or pumping fluids.

The pollution control device is in particular suitable for the treatmentof exhaust from diesel engines in motor vehicles. Examples of such motorvehicles include trains, buses, trucks and ‘low capacity’ passengervehicles. By ‘low capacity’ passenger vehicles is meant a motor vehiclethat is designed to transport a small number of passengers, typicallynot more than 15 persons. Examples thereof include cars, vans andso-called mono-volume cars. The pollution control device is particularlysuitable for the treatment of exhaust from turbo charged directinjection diesel engines (TDI) which are more and more frequently usedin motor vehicles in particular in Europe.

The following examples further illustrate the invention without howeverintending to limit the scope of the invention thereto.

EXAMPLES

Materials Employed in the Examples

S-2 Glass fibers, diameter about 9 μm, chopped to a length of 1.0 inches(25.4 mm), obtainable as 401 S-2 Glass Chopped Strands from AdvancedGlassfiber Yarns LLC (AGY), Aiken, S.C./USA.

E Chopped glass strands, diameter 9 μm, chopped to a length of 1 inch(25.4 mm) from Advanced Glassfiber Yarns LLC (AGY), Aiken, S.C./USA.

R Glass fibers (typical composition 60% SiO₂, 25% Al₂O₃, 9% CaO, and 6%MgO) having a diameter of ca. 10 μm, chopped to a length of 36 mm,available from Saint-Gobain Vetrotex Deutschland GmbH,Herzogenrath/Germany.

Test Methods

Real Condition Fixture Test (RCFT)

This test models actual conditions found in a pollution control devicewith a monolith or diesel particulate trap during typical use, andmeasures the pressure exerted by the mounting material under thosemodelled use conditions. The RCFT method is described in detail inMaterial Aspects in Automotive Pollution control devices, ed. Hans Bode,Wiley-VCH, 2002, pp.—206-208.

Two 50.8 mm by 50.8 mm heated stainless steel platens, controlledindependently, were heated to different temperatures to simulate themetal housing and monolith temperatures, respectively. Simultaneously,the space or gap between platens was increased by a value calculatedfrom the temperature and the thermal expansion coefficients of a typicalpollution control device of the type specified. Normal drivingconditions for the diesel pollution control device are simulated by amonolith temperature of up to 300° C. and a metal housing temperature ofup to 100 and more severe conditions as may occur during continuousdriving at high speed as for example on a motorway were simulated with amonolith temperature of up to 500° C. and a metal housing temperature ofup to 200° C.

Three cycles of the RCFT were performed on each mounting mat sampleusing a 1200-1400 g/m² weight per area mat. The density of the mat whenmounted in the test sample was 0.35 g/cm³. The intumescent comparativeexample was tested at a density of 1.0 g/cc.

After the three RCFT cycles are run, data curves are generated. Thecurves show the pressure between the two plates as a function oftemperature, where the temperatures of the first and second plates,respectively were first increased, held at temperature for 15 minutesand then reduced.

Hot Vibration Test

The hot vibration test was used to further evaluate the suitability ofthe mounting mat according to the present invention as a mounting matfor a low temperature, pollution control device for diesel engines. Thehot vibration test involved passing exhaust gas through a pollutioncontrol device element mounted with a mounting mat in a metallic casing(referred to as a test assembly below) while simultaneously subjectingthe converter assembly to mechanical vibration sufficient to serve as anaccelerated durability test.

The test assembly comprised:

1) a cylindrical ceramic monolith (4.66 inches (11.8 cm) in diameter by3.0 inches (7.6 cm) in length) having 350 cells/in² and wall thicknessof 5.5 mil (0.14 mm),

2) a mounting mat described in the Examples or Comparative Examplesbelow arranged in a cylindrical manner between the monolith and themetal housing and

3) a cylindrical can-shaped housing comprising 409 stainless-steelhaving an inside diameter of approximately 4.88 inches (12.4 cm).

A conventional shaker table (commercially available from Unholtz-DickieCorp. of Wallingford, Conn./USA) was employed to provide vibration tothe test assembly. The heat source comprised a natural gas burnercapable of supplying gas inlet temperature to the converter of up to1000° C. The converter was equipped with a thermocouple to measure theinterface temperature between the outside surface of the monolith andthe inside surface of mounting mat. The exhaust gas temperature wascycled (raised and lowered repeatedly) so as to put extra stress on themounting mat material. A 15-hour thermal conditioning stage was carriedout before the shaking segment of the test was started. The thermalconditioning stage consisted of 5 cycles of two hours at a selectedelevated temperature followed by 1 hour at 50° C. During the shakingsegment of the test, “sine on random” type vibration was employed togenerate further stress and simulate accelerated ageing of the testassembly under use conditions. In a first step, the vibration began at avibration level of 1.75 g (in this vibration test, ‘g’ represents theforce of gravity) on a random 0.01 g²/Hz (approximately 10 g peak). Thevibration was continued for 3 hours at the selected elevated temperatureand then stopped. The test assembly was allowed to cool to 50° C. andheld there for 1 hour without shaking. In a second step, the vibrationlevel was then doubled (i.e. 3.5 g sine on 0.02 g²/hz random) as thetest assembly was heated for an additional 3 hours at temperature. Thevibration was then stopped for a second hour and cooling to and holdingat 50° C. In a third step, the vibration parameters just described weredoubled and the cycle (comprising 3 hrs shaking at the selected elevatedtemperature and 1 hour at 50° C.) was repeated. In a fourth step, thevibration parameters were again doubled, for a total 4 steps, i.e.,until vibration parameters comprising a sine of 28 g's on 0.16 g²/hzrandom (approximately 61 g peak) were attained. The test assembly wassubjected further to the last set of vibration parameters until testassembly failure was noted or until at least 14 cycles at 28 g's sine on0.16 g²/hz random was reached.

Cyclical Compression Test

The test apparatus for the Cyclical Compression Test comprised:

a) a commercially available test instrument (commonly known as a tensiletester) comprising a lower fixed portion and an upper portion movableapart from the lower portion in the vertical direction at a rate definedas the “crosshead speed” and bearing a load cell capable of measuringforces up to 30 kN (MTS™ Model Alliance RT/30, available from MaterialTest Systems, Cary, N.C.),

b) a first quartz tube (50.8 mm in diameter×20 cm long) attached in avertical manner to the fixed lower portion of the instrument,

c) a second quartz tube (50.8 mm in diameter×20 cm long) attached in avertical manner to the load cell on the upper portion of the instrument,

d) a thermocouple extended through the upper quartz tube to make contactwith the test assembly and

e) an electrically heated oven having a brick lining bearing a tubularhole arranged such that it intimately surrounded the portions of the twoquartz tubes nearest one another.

The test assembly consisted of three discs superimposed upon oneanother:

a) a larger lower quartz disc (20.0 mm in thickness, 75 mm in diameter)for supporting the test sample

b) a test sample of a mounting mat to be tested comprising a weigheddisc of the mounting mat to be tested having a diameter of a ca. 2inches (51 mm)

c) a smaller upper quartz disc (12.5 mm in thickness, 51 mm in diameter)located on top of the test sample.

The test assembly was placed between the upper end of the lower quartztube and the lower end of the upper quartz tube in a manner such thatthe three discs of the test assembly were arranged vertically withrelation to each other.

Two gap distances were then selected:

1) Gap 1—a first smaller distance between the two quartz discs

2) Gap 2—a second larger distance between the two quartz discs.

The gap distances were selected so that the mounting mat sample to betested had a density corresponding to the recommended mount density fora given material at the smaller “gap 1” and at the “larger gap 2” adensity of 10% above the density at smaller “gap 1”, these parametersbeing selected based on mat densities commonly encountered when mountingmats are employed under actual use conditions.

The two gap distances thus selected were then programmed into theinstrument, the oven was closed around the test assembly and heated toand held at 250° C., and finally the instrument was programmed torepeatedly move from one gap distance to the other, thus repeatedlyincreasing and decreasing pressure on the sample disc located in thetest assembly between the two quartz discs. The cross-head speed was 5.0mm/min and there was essentially no dwell time at either of the “gap 1”or “gap 2” positions.

The pressure exerted by the sample disc at any one time was recorded inunits of kilo Pascal (kPa). The compression cycle was repeated 1000times.

The pressure exerted by the sample disc at the beginning of the test inthe (initial pressure) was recorded while the instrument was in thesmaller “gap 1” location of the compression cycle. The pressure exertedby the sample disc after 1000 compression cycles at 250° C. (finalpressure) was also recorded, again while the instrument was in thesmaller “gap 1” location of the compression cycle.

These two numbers were compared in the following manner: (finalpressure/initial pressure)×100%=percent retention.

Example 1

40 liters of S-2 glass fibers of approximately 9 μm in average diameterand 2.54 cm in length were obtained from Advanced Glassfiber Yarns LLC(AGY). The fibers were essentially shot free.

The glass fibers were opened in a two-zone Laroche opener. The firstzone had a feed speed of 2 m/min and a Lickerin roll speed of 2,500rev/min. The second zone had a feed speed of 4 m/min and a Lickerin rollspeed of 2,500 rev/min. The output speed was 6.5 m/min.

The opened fibers were then fed into a conventional web-forming machine(commercially available under the trade designation “Rando Webber” fromRando Machine Corp. of Macedon, N.Y., wherein the fibers were blown ontoa porous metal roll to form a continuous web. The continuous web wasthen needle-bonded on a conventional needle tacker. The needle speed was100 cycles/min and the output speed was 1.1 m/min. The “weight per area”of the mounting mat could be adjusted as desired. In tests where thevalue of “weight per area” substantially influences the test results,this parameter is indicated along with the test results. The compositionof the mounting mat of Example 1 is summarized in Table 1 below.

The mat of Example 1 was tested according to the RCFT method describedabove under Test Methods. A family of three data curves was generated,representing each of three cycles. The mounting mats of Example 1displayed a very uniform pressure over the temperature range examinedand provided a pressure well above the minimum pressure (about 40 kPa)needed to hold the monolith securely in place. The RCFT data for Example1 are shown in FIG. 2. In FIG. 2, the X-axis represents the temperaturescale for the simulated monolith temperature and the simulated skintemperature. For the monolith temperature, the temperature rangerepresented in FIG. 1 was from 20 to 300° C. at the line indicated with‘A’ and from 300 (indicated by the line ‘B’) back to 50° C. For thesimulated skin temperature, the ranges were respectively 20 to 100° C.and 100° C. to 25° C. The interval between lines A and B indicates aperiod of 15 minutes for which the sample was held at the maximumtemperature. The Y-axis represents the pressure measured. The scale wasfrom 0 to 500 kPa. Curve 1 to 3 represent the results of the 1^(st) to3^(rd) cycle respectively. An adequate holding force over the entiretemperature range tested was found as can be seen in FIG. 2.

The mat of Example 1 was also subjected to the Hot Vibration Test asdescribed above under Test Methods. The Hot Vibration Test was performedat two temperatures: 300° C. and 500° C., respectively. At 300° C., themat of Example 1 had not failed after 72 hours. At 500° C., Example 1had not failed after 80 hours. Hot Vibration Test results are summarizedbelow in Table 2.

Example 2

Example 2 was prepared by the method described in Example 1 with theexception that E-glass fibers (chopped glass strands, diameter 9 μm,chopped to a length of 1 inch (25.4 mm) available from AdvancedGlassfiber Yarns LLC (AGY), Aiken, S.C., USA) were employed. Thecomposition of the mat of Example 2 is summarized in Table 1.

Tests on the mat of Example 2 include the Cyclical Compression Test.Results are summarized in Table 3 and show that at diesel pollutioncontrol device temperatures (i.e., average mat temperature of 250° C.),the mat keeps 86.3% of its original pressure after 1000 compressioncycles.

Example 2 was also tested in the RCFT using the same conditions as usedin Example 1. Example 2 maintained adequate holding force over theentire temperature range.

Example 3

R-glass fibers (60% SiO₂, 25% Al₂O₃, 9% CaO, and 6% MgO) having adiameter of ca. 10 μm, chopped to a length of 36 mm, obtained fromSaint-Gobain Vetrotex, were processed into a web by the method describedin Example 1. The composition of the mat of Example 4 is summarized inTable 1.

Tests on the R-glass mat of Example 3 include the Cyclical CompressionTest. Results are summarized in Table 3 below and show that at dieselpollution control device temperatures (i.e., average mat temperature of250° C.), the mat keeps 95.5% of its original pressure after 1000compression cycles.

Additionally, an RCFT test was performed on the mat of Example 3 in thesame way as for the mat of Example 1 except that the simulatedtemperature range for the monolith was from 25° C. to 500° C. and 25 to200° C. for the skin. Example 3 maintained adequate holding force overthe entire temperature range.

Example 4

A two layer mat was prepared by laminating two separately preparedlayers together. The first layer comprised R-glass. The second layercomprised E-glass. The two layers were put together by needle-bonding.The mounting mat formed in this manner had two discrete layers of glassof differing compositions. The composition of the two layer mat ofExample 4 is summarized in Table 1.

The two layer mat of Example 4 was subjected to the RCFT Test using thetemperature conditions of Example 3. The mounting mat of Example 4maintained adequate holding force over the entire temperature range.

Example 5

A two layer mat was prepared by laminating two separately preparedlayers together. The first layer comprised S-2 glass. The second layercomprised E-glass. The two layers were put together by needle bonding.The mounting mat formed in this manner had two discrete layers of glassof differing compositions. The composition of the mat of Example 5 issummarized in Table 1.

The mat of Example 5 was subjected to the Cyclical Compression Test.Results are summarized in Table 3 and show that at diesel convertertemperatures (i.e., average mat temperature of 250° C.), the mat keeps82.2% of its original pressure after 1000 compression cycles.

Comparative Example 1

Comparative Example 1 (C1) comprised a binder-free, non-woven fiber matmade of Belcotex silica fibers having a fiber diameter of 9 microns,obtained from Belchem Fiber Materials GmbH, Brand-Erbisdorf, Germany.

This material was subjected to the Real Conditions Fixture Test (RCFT)at a mount density of 0.4 g/cm³. The temperature ranges used, were thesame as used in Example 3 above. Test results are shown in FIG. 3 by thecurve C1. FIG. 3 shows third cycle results. The mat of Example 1 wassubjected to the same conditions as used in the RCFT test for the mat ofthis Comparative Example 1 and is shown as curve 1 in FIG. 3. It can beseen from FIG. 3 that the mat of Comparative Example 1 did not maintainsufficient pressure to hold the monolith in place under the simulatedconditions whereas the mat of Example 1 maintained sufficient pressureto hold the monolith.

Comparative Example 2

Comparative Example 2 (C2) comprised a non-woven, binder-free fiber matmade of silica fibers commercially available under the name Silcosoft™from BGF industries in Altvista, Va. The fibers in the mat have anaverage diameter of 9 microns.

This material was subjected to the Real Conditions Fixture Test (RCFT)using the simulated temperature conditions of Comparative Example 1 at amount density of 0.4 and 0.45 g/cm³. Test results for the third cycleare shown in FIG. 3 as curve C2a (0.45 g/cm³ mount density) and C2b(0.40 g/cm³ mount density), and show that the mat of Comparative Example2 also did not maintain sufficient pressure, to hold the monolith inplace under conditions simulating these encountered with a dieselengine. The Y axis in FIG. 3 representing the pressure had a scale of 0to 600 kPa.

Comparative Example 3

Comparative Example 3 (C3) comprised a commercially availableintumescent pollution control device mounting mat. It comprises about55% unexpanded vermiculite, about 37% fiber, and about 8% organicbinder. The fibers are melt-formed, amorphous, shot-containingalumina/silica fibers having a diameter of about 2-3 microns having alength of not more than 0.5 inch.

Comparative Example 3 was tested according to the Hot Vibration Test at300° C. The sample failed after 8 hours. Hot Vibration Test results aresummarized in Table 2 Comparative Example 3 was also tested in theCyclical Compression Test. Results are shown in Table 3 and show that atdiesel pollution control device temperatures (i.e., average mattemperature of 250° C.), the mat keeps only 25.3% of its originalpressure after 1000 cycles which is unacceptably low.

Comparative Example 3 was also tested in the RCFT test using thetemperature conditions of Example 1. An unacceptably low holding forcealready after the first cycle was noted.

Comparative Example 4

Comparative Example 4 (C4) comprised a reduced vermiculite intumescentpollution control device mounting mat that is commercially available. Itcomprised about 37% unexpanded vermiculite, about 54% fiber, and about9% organic binder. The fibers were the same as those of ComparativeExample 3.

Comparative Example 4 was tested according to the Hot Vibration Test atboth 300° C. and 500° C. The sample failed after 8 hours at 300° C. andfailed after 18 hours at 500° C. Hot Vibration Test results aresummarized in Table 2.

Comparative Example 5

Comparative Example 5 (C5) comprised a wet laid mat prepared frommelt-formed, amorphous, shot containing alumina silicate fibersavailable as Kaowool Bulk Fibers from Thermal Ceramics in Augusta, Ga.The fibers have a diameter of 2-3 microns and a length of about 0.5inch.

Comparative Example 5 was tested according to the Cyclical CompressionTest described above under Test Methods and exhibited a percentretention of 49.8% which is unacceptable for use in a pollution controldevice for diesel engines. Cyclical Compression Test results aresummarized in Table 3.

Comparative Example 6

Comparative Example 6 (C6) comprised “a non-woven binder-free matmaterial” commercially available from Thermal Ceramics UK Lmtd. Wirrel,Merseyside, England as Ultrafelt™ Paper having a density of 12 lb/ft3(0.2 g/cm³) The mat is a needle bonded mat of alumina/silica fibers (47%Al₂O₃ and 53% SiO₂). According to the technical data sheet of themanufacturer of the mat, the fibers would have a length that is longerthan typically used in paper making. This would indicate the fiberswould have a length of more than 0.5 inch.

Comparative Example 6 was also tested according to the CyclicalCompression Test described above under Test Methods and exhibited apercent retention of 41.2%, which is unacceptable low.

Cyclical Compression Test results are summarized in Table 3.

Comparative Example 7

Comparative Example 7 (C7) comprised a commercially availablenon-intumescent pollution control device mounting mat made of highalumina polycrystalline ceramic fibers. The fibers are essentiallyshot-free and have an average diameter of 3 microns.

Comparative Example 7 was also tested according to the CyclicalCompression Test described above under Test Methods and exhibited apercent retention of 81.1%. Cyclical Compression Test results aresummarized in Table 3. Retention force of these very expensive fiberswere acceptable, but not as good as the present invention. TABLE 1Summary of compositions of mounting mats according to the inventionExample Glass type Fiber diameter, μm Fiber length, mm 1 S-2  9 25.4 2 E10 25.4 3 R 10 36 4 R + E R = 10 R = 36 Two layer E = 10 E = 25.4 5S-2 + E S-2 = 9 S-2 = 25.4 Two layer E = 10 E = 25.4

TABLE 2 Summary of results from the hot vibration test Peak Peak MountDensity Temp. Vibration Total No. Ex. (g/cm³) (° C.) (g²/Hz) CyclesResults 1 0.32 300 0.16 23 No failure after 72 hours 1 0.32 500 0.16 25No failure after 80 hours C3 1.04 300 0.04 2 Failed after 8 hours C40.81 300 0.04 2 Failed after 8 hours C4 0.85 500 0.16 4 Failed after 18hours

TABLE 3 Summary of cyclical compression test results Percent retentionafter Example Mounting mat type 1000 cycles at 250° C. 2 Single layer E86.3% 3 Single layer R 95.5% 5 One layer E, one layer S-2 82.2% C3intumescent mat 25.3% C5 TC HA-Bulk 49.8% C6 Ultrafelt ™ 41.2% C7Polycrystalline Mat 81.1%

1. Pollution control device comprising a pollution control elementarranged in a casing with the mounting mat according to claim
 12. 2.Pollution control device according to claim 1 wherein said glass fiberscomprise aluminium oxide in an amount of 10 to 30% by weight, silicondioxide in an amount of 52 to 70% by weight and magnesium oxide in anamount of 1 to 12% by weight based on the total weight of the glassfiber and wherein the weight percentages of aluminium oxide, silicondioxide and magnesium oxide are calculated on a theoretical basis asAl₂O₃, SiO₂ and MgO respectively.
 3. Pollution control device accordingto claim 1 wherein glass fiber compositions differ in the length ofglass fiber and/or the average diameter of the glass fiber.
 4. Pollutioncontrol device according to claim 1 wherein the glass fiber compositionsdiffer in the chemical composition of the glass fiber.
 5. Pollutioncontrol device according to claim 1 wherein the glass fibers areselected from the group consisting of E-glass fibers, S-glass fibers,S-2 glass fibers, R-glass fibers and a mixture thereof.
 6. Pollutioncontrol device according to claim 1 wherein one of said at least twolayers contacts said casing and comprises E-glass fibers, and another ofsaid at least two layers contacts said pollution control element andcomprises at least one of S-glass fibers, S-2 glass fibers, R-glassfibers and a mixture thereof.
 7. Pollution control device according toclaim 1 wherein the mount density of said mat is between 0.2 and 0.7g/cm³.
 8. Machine comprising a diesel engine and a pollution controldevice as defined in claim
 1. 9. Machine according to claim 8 whereinsaid machine is a motor vehicle and said diesel engine is a turbocharged direct injection diesel engine.
 10. Machine according to claim 8wherein said machine is a motor vehicle selected from a truck, a bus ora low capacity passenger vehicle.
 11. Method of treating exhaust gasfrom a diesel engine by subjecting the exhaust gas to a pollutioncontrol device as defined in claim
 1. 12. A mat for mounting a pollutioncontrol element in a casing of a pollution control device, said matbeing a non-intumescent mat comprising at least 90% by weight based onthe total weight of the mat of chopped magnesium aluminium silicateglass fibers that have a number average diameter of 5 μm or more and alength of 0.5 to 15 cm, said glass fibers being needle punched or stitchbonded, said mat being free or substantially free of organic binder andbeing comprised of at least two layers of said chopped magnesiumaluminium silicate glass fibers, wherein said at least two layers differin their magnesium aluminum silicate glass fiber composition.
 13. Mataccording to claim 12 wherein said glass fiber compositions differ inthe length of glass fiber and/or the average diameter of the glassfiber.
 14. Mat according to claim 12 wherein said glass fibercompositions differ in the chemical composition of the glass fibers. 15.Mat according to claim 12 wherein said glass fibers comprise aluminiumoxide in an amount of 10 to 30% by weight, silicon dioxide in an amountof 52 to 70% by weight and magnesium oxide in an amount of 1 to 12% byweight based on the total weight of the glass fiber and wherein theweight percentages of aluminium oxide, silicon dioxide and magnesiumoxide are calculated on a theoretical basis as Al₂O₃, SiO₂ and MgOrespectively.
 16. Mat according to claim 12 wherein the glass fibers areselected from the group consisting of E-glass fibers, S-glass fibers,S-2 glass fibers, R-glass fibers and a mixture thereof.
 17. Mataccording to claim 16 wherein one of said at least two layers contactssaid casing and comprises E-glass fibers, and another of said at leasttwo layers contacts said pollution control element and comprises atleast one of S-glass fibers, S-2 glass fibers, R-glass fibers and amixture thereof.
 18. Mat according to claim 13 wherein the glass fibercompositions also differ in the chemical composition of the glass fiber.19. Pollution control device according to claim 3 wherein the glassfiber compositions also differ in the chemical composition of the glassfiber.
 20. Pollution control device according to claim 19 wherein theglass fibers are selected from the group consisting of E-glass fibers,S-glass fibers, S-2 glass fibers, R-glass fibers and a mixture thereof.